1. Field of the Invention
This invention relates to novel antioxidant-peroxides (AO-P's) and the use of AO-P's for producing polymeric compositions of enhanced oxidative stability via initiation of polymerization of ethylenically unsaturated monomers, curing of elastomers and unsaturated polyester resins, modification of polypropylene and propylene copolymers, and crosslinking of ethylene polymers and copolymers.
2. Description of the Prior Art
Many polymers undergo oxidative degradation to varying degrees during processing fabrication and/or during the intended lifetime of the articles fabricated from them. Typically, antioxidant compounds are added to polymeric materials in order to protect them during processing as well as during their use. Generally, such antioxidant compositions consist of one or more compounds selected from hindered phenols, sulfides, amino, hydrazido, and phosphorous III (phosphites, phosphonites, etc.) compounds. In many cases combinations of hindered phenols and sulfides exhibit synergy when used with certain polymeric systems, (Kirk-Othmer Encyclopedia of Chemical Technology, "Antioxidants and Antiozonants," Third Edition, Vol. 3, pp. 128-148 (1978)). The same reference describes various types of antioxidants that are employed for suppressing the oxidation of foods, petroleum products and polymers (natural, as well as synthetic). This reference also describes the mechanisms of antioxidant actions.
However, not all antioxidants operate by the same antioxidant mechanism. Hindered phenol antioxidants such as 2,6-di-t-butyl-4-methylphenol and secondary arylamines react rapidly with reactive free radicals on the backbone of the polymer and thus prevent the formation of labile hydroperoxide groups along the backbone of the polymer chain. Dialkyl sulfides (e.g., dialkyl thiodipropionates), trialkyl and triaryl phosphites and metal salts of sulfur compounds (e.g., zinc dithiocarbamates, zinc dithiophosphates and zinc xanthates) suppress oxidation of polymers by reacting with hydroperoxy groups, thus producing non-peroxidic species.
Many of the antioxidants developed commercially are fugitive in nature, that is, they are extractible from the compositions with which they are used, owing to their relatively low molecular weights and the relatively high temperatures required for processing the polymers in which they are employed. In addition, many potential antioxidants are easily extracted by solvents from the host polymer.
There has been considerable effort in recent years to develop non-fugitive antioxidants. Such antioxidants are referred to as non-fugitive antioxidants, since they do not leach out or are not extractible from the polymer or other composition to which they are added, either during processing or during the end use of the composition.
One of the reasons for this emphasis is the need to fabricate polymeric articles that must pass stringent United States Food and Drug Administration extraction tests in order to qualify the polymeric composition for use in food and drug applications. Another reason for non-fugitive antioxidants is the trend to higher processing temperatures which results in larger losses of conventional lower molecular weight antioxidants. Losses during polymer processing results in greater costs for the antioxidant and exposes workers, the public and the environment to possibly toxic materials.
The main approaches to achieving non-fugitive antioxidants were to significantly increase the molecular weight of the antioxidant by attaching several antioxidant moieties to one compound or by attaching antioxidant moieties to high molecular weight compounds (e.g., polymers).
One approach to making antioxidants non-fugitive is to covalently bond the antioxidant to the polymer. J. A. Kuczkowski and J. G. Gillick, "Polymer-Bound Antioxidants", Rubber Chemistry and Technology, Vol. 57, pages 621-651 (1984), disclose approaches for attaching of antioxidants to polymers. The approaches include preparation, polymerization and copolymerization of antioxidants possessing free radically polymerizable, ethylenically unsaturated groups and reaction between antioxidants and polymers possessing co-reactive groups.
The Kirk-Othmer Encyclopedia of Chemical Technology Third Edition, Vol. 17, "Peroxides and Peroxy Compounds, Organic", pp. 27-90 (1982), also describes various types of organic peroxides and peroxy compounds and their uses for producing and modifying polymeric materials.
T. H. Fisher and J. C. Martin, J. Am. Chem. Soc. 88, p. 3382 (1966), reports the decomposition kinetics for t-butyl peroxy-2-(phenylthio)benzoate and t-butyl peroxy-2,6-di-(phenylthio)benzoate. Owing to the proximity of the sulfur group to the peroxide group, accelerated rates of decompositions were observed for these peroxyesters compared to the analogs without sulfur groups. The use of these sulfur-containing peroxyesters as AO-P's was not suggested by the authors.
Diperoxyketals possessing sulfide (--S--), sulfoxide (--SO--) and sulfone (--SO.sub.2 --) moieties are disclosed in U.S. Pat. No. 3,488,392. The structure of the novel AO-P's of this invention does not cover these sulfur containing peroxyesters or diperoxyketals.
The chemical literature does not disclose the preparations of antioxidant-peroxides (AO-P's) of the hindered phenol type and the use of these novel compounds for producing polymeric compositions of enhanced oxidative stability via initiation of polymerization of ethylenically unsaturated monomers, curing of elastomers and unsaturated polyester resins, modification of polypropylene and propylene copolymers, and crosslinking of ethylene polymers and copolymers. It is surprising that the novel AO-P's of the present invention are useful, dual functional compounds in view of the fact that the organic peroxide and the antioxidant are at cross purposes with each other (generation of free radicals and deactivation of free radicals, respectively). One skilled in the art would be discouraged to prepare and find useful compounds and compositions with antioxidant and peroxide functions in the same molecule. In these processes the antioxidant becomes covalently bound to the polymer thereby making the antioxidant non-fugitive.